The present invention relates to an information recording medium for optical disks and to a recording apparatus to make records thereon.
There are many known principles of recording information on a thin film (recording film) by irradiating it with a laser beam. One of them utilizes the phase transition (or phase change) of film material or the photo darkening, which are due to the change in atomic arrangement induced by irradiation with a laser beam. It offers the advantage of causing almost no deformation to the thin film and hence gives an information recording medium of double-sided disk structure which is formed by bonding together two disks.
Such an information recording medium is usually made up of a substrate, a protective layer, a recording film of GeSbTe or the like, a protective film of ZnS--SiO.sub.2, and a reflective layer. The recording film is higher in reflectivity when it is in the crystalline state than when it is in the amorphous state. Therefore, the recording film has a greater absorptance when it is in the amorphous state. If over-write is carried out in this state, the amorphous region increases in temperature faster than the crystalline region due to recording and hence the newly formed record marks become large enough to distort reproduced signals.
Attempts have been made to eliminate this drawback by making the recording film to increase in absorptance more when it is in the crystalline state than when it is in the amorphous state. For example, literature (1) mentions the reversion of absorptance by providing a very thin (10 nm) reflective layer of Au. (Yamada and other three, Shingaku Gihou. MR92-71, CPM92-148 (1992-12) p. 37) Further, literature (2) mentions the reversion of absorptance by using a 65-nm thick silicon film as the reflective layer. (Okada and other six, Shingaku Gihou. MR93-53, CPM93-105 (1993-12) p. 1)
It is known from accelerated tests that the protective layer of this kind poses a problem with deterioration of record marks when high-density recording is carried out or when the recording medium is stored at room temperature or apparatus temperature for 10 years or more.
In this specification, the term "phase change" implies not only the phase change between the crystalline state and the amorphous state but also the phase change that takes place at the time of melting (change into liquid phase) and recrystallization and the phase change between the crystalline state and the crystalline state. The flow of the recording film occurs as the recording film flows due to irradiation with a laser beam at the time of recording and also as the recording film is gradually pushed by the deformation (thermal expansion) of the protective layer and intermediate layer. The term "mark edge recording" means the recording system which makes the edge of the record mark to correspond to the signal "1" and the space between marks and the mark inside to correspond to the signal "0".
Conventional information recording media suffer the disadvantage that jitter increases when rewriting is repeated frequently, record marks deteriorate during storage, and reflectivity level fluctuates in the case where they are used as high-density, rewritable information recording media of phase transition type that employ the mark edge recording system.